The present invention relates to a differential amplifier circuit, and more specifically relates to stabilization technology of an output voltage of the amplifier circuit.
Referring to FIG. 1, a conventional differential amplifier circuit includes a differential amplifier unit having an emitter circuit comprised of transistors Q1 and Q2. A current mirror circuit comprised of transistors Q3 and Q4 is used as a current source for the emitter circuit. As shown in FIG. 1, a current I.sub.0 flowing into the current mirror circuit is controlled by a circuit comprised of a transistor Q5, resistors R7, R8 and a diode ZD.sub.1.
FIG. 2 shows another example of a conventional differential amplifier circuit, in which a current I.sub.0 flowing into the same type of a current mirror circuit is controlled by a circuit comprised of transistors Q4 and Q5, resistors R6, R7 and R8, and a diode ZD.sub.2.
In either of the two conventional circuits, the current I.sub.0 flowing through the resistor R7 is held constant regardless of a variation in a power source voltage Vp. This current I.sub.0 drives the current mirror circuits. In FIGS. 1 and 2, V.sub.n indicates a negative power source.
These conventional differential amplifier circuits are controlled such that the current flowing into the differential amplifier unit is held constant even when the power source voltage Vp varies. Consequently, a voltage drop across output resistors R1 and R2 of the differential amplifier unit is kept constant. Therefore, when the positive power source voltage Vp varies, the voltage variation directly appears in outputs +V.sub.0 and -V.sub.0 of the differential amplifier unit. Generally, the output of the differential amplifier unit is utilized in the form (+V.sub.0)-(-V.sub.0), hence the above noted variation of the power source voltage contained in +V.sub.0 and -V.sub.0 cancel each other, thereby causing no drawback. However, in some applications in which the differential amplifier circuit may be utilized, for example as a comparator, only one of the outputs is used. In such a case, a DC offset of the output may vary disadvantageously with the variation in the power source voltage. Further, when the power source contains noise, disadvantageously the noise would be directly outputted.